1. Technical Field
The present disclosure relates generally to the field of well logging. More specifically, the subject matter of this disclosure relates to techniques for using log data for formation evaluation.
2. Background Information
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions of prior art.
Logging tools may be used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, acoustic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline (WL) tools are still used extensively. However, the need for real-time or near real-time information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. By collecting and processing such information during the drilling process, the driller may modify or correct key steps of the well operations to optimize drilling performance and/or well trajectory.
MWD tools typically provide drilling parameter information such as weight-on-bit, torque, shock and vibration, temperature, pressure, rotations-per-minute (rpm), mud flow rate, direction, and inclination. LWD tools typically provide formation evaluation measurements such as natural or spectral gamma ray, resistivity, dielectric, sonic velocity, density, photoelectric factor, neutron porosity, sigma thermal neutron capture cross-section (E), a variety of neutron induced gamma ray spectra, and nuclear magnetic resonance (NMR) distributions. MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas or sensors in general), but MWD and LWD tools may be constructed to not only endure but to operate in the harsh environment of drilling. The terms MWD and LWD are often used interchangeably, and the use of either term in this disclosure will be understood to include both the collection of formation and wellbore information, as well as data on movement and placement of the drilling assembly.
Logging tools may be used to determine formation volumetrics, that is, quantify the volumetric fraction, usually expressed as a percentage, of each and every constituent present in a given sample of formation under study. Formation volumetrics involves the identification of the constituents present, and the assigning of unique signatures for constituents on different log measurements. When, using a corresponding earth model, all of the forward model responses of the individual constituents are calibrated, the log measurements may be converted to volumetric fractions of constituents. Some examples of techniques for determining formation volumetrics are disclosed in U.S. patent application Ser. No. 13/837,409 (filed Mar. 15, 2013) and Ser. No. 13/836,651 (filed Mar. 15, 2013), both of which are commonly assigned.
In mature water flooded reservoirs, certain properties, such as formation water salinities, can vary considerably. If ignored, these variations can lead to erroneous water saturation (Sw) and high uncertainty in reserve calculations. Further, variations in water flooding salinities can alter some petrophysical parameters and can cause them to behave unexpectedly in some cases (i.e., in a non-Archie manner). Existing formation evaluation techniques in variable water salinity environments typically require accurate knowledge of at least some formation matrix and fluid petrophysical properties. However, in many real world applications, these properties may not always be available and, in some cases, are assumed and may not be accurate.